The actin assembly requirements of the formin Fus1 to build the fusion focus

Formins form the largest family of actin filament nucleators and elongators, involved in the assembly of diverse actin structures. Actin filament nucleation and elongation activities reside in the formin homology 1 (FH1) and FH2 domains, common to all formins. However, the rate of these reactions varies between formins by at least 20-fold. Typically, each cell expresses several distinct formins, each contributing to the assembly of one or several actin structures, raising the question of what confers each formin its specificity. Here, using the formin Fus1 in the fission yeast Schizosaccharomyces pombe, we systematically probed the importance of formin nucleation and elongation rates for function in vivo. Fus1 assembles the actin fusion focus, an aster-like structure of actin filaments at the contact site between gametes, necessary for the process of cell fusion to form the zygote during sexual reproduction. By constructing chimeric formins with combinations of FH1 and FH2 domains previously characterized in vitro, we establish that changes in formin nucleation and elongation rates have direct consequences on the architecture of the fusion focus, and that Fus1 native high nucleation and low elongation rates are optimal for fusion focus assembly. We further describe a point mutant in the Fus1 FH2 domain that preserves native nucleation and elongation rates in vitro but alters function in vivo, indicating an additional property of the FH2 domain. Thus, rates of actin assembly are tailored for assembly of specific actin structures.